1. Field of the Invention
This invention relates to a ferroelectric memory using ferroelectric material as information storing medium (memory) and methods for driving and manufacturing the same.
2. Description of the Related Art
It is generally known that ferroelectric material has a hysteresis characteristic and data can be stored by utilizing the hysteresis characteristic. FIG. 58 shows the hysteresis characteristic, in which the abscissa indicates electric field strength E and the ordinate indicates polarization intensity P. In the drawing, Ec denotes an electric field causing a polarization direction to be inverted, or electric field (referred to as a coercive electric field) occurring when the polarization value is set at "0", and Es denotes an electric field (referred to as an inverted electric field) causing the direction of the hysteresis loop to be inverted. As shown in FIG. 58, when the electric field strength is 0, the polarization is set into one of two states A and C which are respectively set to correspond to digital signals "1" and "0". That is, when the polarization is set in state A, digital signal "1" is stored, and when the polarization is in state B, digital signal "0" is stored.
Assume that signal "1" is stored in the ferroelectric material and the polarization is in state A. If, in this condition, positive going readout pulse Ec is applied to the ferroelectric material, the polarization state is changed from A to B, and is set back to A again. At this time, variation in the polarization intensity for the variation amount of Er is small so that variation in capacitance C.sub.L associated with the ferroelectric material may be small. In contrast, when signal "0" is stored in the ferroelectric material and the polarization is in state B and if positive going readout pulse Ec is applied, the polarization state is changed from C to D, and is set back to C again. Since, at this time, variation in the polarization intensity between polarization states C and D is large, causing variation in capacitance C.sub.L to be large. In this way, when signal "1" is stored, the capacitance variation is small and a small output is derived, and when signal "0" is stored, the capacitance variation is large and a large output is derived. Thus, the "1" and " 0" states can be determined according to the output and as a result data can be read out.
As is seen from FIG. 58, a recording pulse of voltage level Es may be applied in order to change the polarization state of the ferroelectric material from "0" to "1", and a pulse of voltage level -Es may be applied in order to change the polarization state from "1" to "0".
The prior art technique of using the ferroelectric material as information recording medium by utilizing the hysteresis characteristic of the ferroelectric material is disclosed in, for example, Japanese Patent Disclosure Nos. 55-126905, 57-117186, 59-215096 and 59-215097. Another method of recording and reading out information with respect to the ferroelectric material is disclosed in, for example, Japanese Patent Disclosure No. 59-215096. That is, as shown in FIG. 59, photoconductive film or ferroelectric thin film 74 is arranged between transparent electrode 73 which is disposed on substrate 72 and transparent electrode 75 to constitute ferroelectric memory 71. With a voltage kept applied to ferroelectric memory 71, a light beam is applied to a selected portion of the photoconductive film to polarize the selected portion so as to record information. In the readout mode, the information can be read out by applying a light beam to the selected portion by utilizing variation in the refraction, interference and polarization of light due to the polarization of the selected portion.
In the above conventional example, a light beam is applied to the surface of the ferroelectric memory and controlled to sequentially record or read out information by directly applying a light beam to the surface of the photoconductive film of the ferroelectric memory. In this case, the information storing position is not specified on the ferroelectric memory and the storing position is determined by mechanically controlling the position of application of the light beam. For this reason, it is necessary to precisely control the position of application of the light beam so as not be erroneously record information on or read out information from a next storing section because of deviation of the light beam or the like. As the storing density is made higher, the position control must be effected more precisely. Therefore, the construction becomes more complicated. Further, since information can be recorded only in a two-dimensional area, it is difficult to increase the storing capacity by laminating the ferroelectric memories.